Materials having improved nonfouling characteristics and method of making same

ABSTRACT

A method of improving the separatory properties of membranes by the deposition of a fluorinated amphiphilic compound in an oriented Langmuir-Blodgett layer on the membranes surface so as to increase membrane selectivity and counteract membrane surface properties leading to fouling during liquid-liquid separations and enhance gas selectivities of membranes used for gas-gas separations. The use of a fluorinated long-chain pyridinium bromide is specifically disclosed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to liquid purification or separation and, moreparticularly, to the deposition of oriented monolayers on the surface ofseparation membranes.

2. Description of the Prior Art

The cost and energy effectiveness of membrane separation processes areseriously compromised by the readiness with which available membranesundergo fouling by colloidal materials. The anion-exchange membranes ofelectrodialysis (ED) stacks and the incharged membranes ofreverse-osmosis (RO) systems are especially prone to fouling.

The heart of every modern electrodialytic treatment system is analternating array of polymer-based ion-exchange membranes. A seriousobstacle to cost-effective operation of electrolytic desalination plantsis the ease with which these membranes undergo concentrationpolarization and fouling by humic acids. These end products ofbiodegradation are present in most natural waters as colloidal materialsbearing partially ionized acid groups. Their negative character rendersthem much more likely to adhere to a positively charged anion-exchangemembrane than to a cation-exchange membrane, and this adherence has twodeleterious effects: first, the pores of the membrane become physicallyoccluded by colloidal material; and second, the positive bulk with anegative fouled surface functions as a bipolar "sandwich membrane",greatly enhancing its tendency to undergo further fouling.

During continuous operation, these insoluble impurities occlude themembrane surfaces at an increasing rate, and the electrical resistanceof a stack is raised to the point where power costs make furtheroperation uneconomic. The stack must then be disassembled for stringentcleaning or replacement of membranes. The combined expenses ofdown-time, replacement, or cleaning, and power requirements that risesteadily during operation seriously compromise the cost effectiveness ofthis method of water purification.

Fouling of RO membranes proceeds by a less well known, but relatedpattern, in which colloidal materials are occluded on the workingsurfaces of the membranes almost immediately after operation has beeninitiated. In RO separations, greatly reduced membrane flux is thenegative economic factor.

Considerable evidence indicates that a propensity toward polarizationand fouling is governed by the nature of a membrane surface. Criticalsurface characteristics have been shown to include rugosity (roughness),chemical homogeneity, and hydrophilicity. Their demonstrated importanceindicates that surface modification may offer a fruitful avenue to amechanistic definition of concentration polarization and fouling, and totheir mitigation.

SUMMARY OF THE INVENTION

A membrane surface is modified by coating with individually orientedlayers of amphiphilic molecules, i.e., molecules with one polar orhydrophilic end and one non-polar or hydrophobic end, using theclassical Blodgett dipping technique. Because the layers are extremelythin (around 20 Å), they can modify the surface characteristics of asemipermeable selective membrane which lead to fouling without affectingits bulk properties, i.e., the separatory action of the membrane.Membrane surface so treated are physically smooth (the deposited layershaving strong lateral cohesive forces), chemically homogeneous andhydrophobic. The treatment is to be carried out after manufacture of themembrane under nonfouling conditions and before its exposure to foulingconditions.

The types of amphiphilic molecules which are best utilized in thepresent invention include fluorinated compounds (bearing a polar groupat the end of the chain opposite to the fluorinated group). Afluorinated long-chain pyridinium bromide, hereafter referred to asR_(f) PyrBr, was tested extensively as a modifier of the surfaces ofanion-exchange membranes and found to be very effective in preventingfouling. For maximum benefit, the compounds must be applied to surfacesas Langmuir-Blodgett layers so that the resulting layer ismonomolecular, free of defects and strictly oriented with, in the caseof fluorinated amphiphilic compounds, the fluorinated ends facing thefeed solution.

Fluorinated polymerizable materials that can be deposited in the monomerform and polymerized on the surface in such a manner that theorientation of the molecules is retained may be even more effective, inthat the lifetime of the coating may be increased by entanglement withthe original surfaces.

Most non-fluorinated amphiphilic compounds will not be useful in theinvention. The determining factors for prevention of fouling are thatthe compound must exhibit either a neutral charge or a charge identicalto that of the membrane, and that it must be fluorinated. Presumably,identical charge and the greatest possible extent of fluorinationcompatible with the Blodgett transfer technique are most desirable. Theonly criterion for matching membranes with fouling-preventive(fluorinated) amphiphilic compounds is the avoidance of opposingcharges.

Many amphiphilic molecules, nonfluorinated and with charges opposite tothat of the membrane, will lead to greatly enhanced fouling, as isdemonstrated by the experiments set forth herein with arachidic acidcoated membranes. This effect is also consistent with the currentunderstanding of fouling, which is said to be irreversibly initiated bythe very first molecular layer of oppositely charged surface-activematerial that contacts the membrane.

Molecular weight of the amphiphilic compound is of great significance inthat it governs, to a large extent, the surface-active character of thecompounds to be used. Molecular weight should probably range between 350and 700 for fluorinated compounds, depending on the nature of the polargroup and the atomic weight of a counterion, if there is one. Thecompounds must be virtually insoluble when delivered to a water surface(by the standard Blodgett technique) as a very dilute solution in awater-immiscible low-boiling solvent. To some extent, slight solubilitycan be compensated for by lowering the deposition temperature, as wasdone for R_(f) PyrBr.

In summary, the amphiphilic compounds that are useful in the presentinvention are fluorinated, surface-active, neutral or charged likemembrane and sufficiently water-insoluble at the temperature andpressure of transfer to be amenable to deposition as Langmuir-Blodgettlayers.

The deposition of R_(f) PyrBr was carried out at the lowest temperaturereading obtainable under the laboratory conditions, 10.5° C. It isprobable that a still lower temperature would further decrease the watersolubility of R_(f) PyrBr, which is desirable. Therefore, a range of 1°C. to 10° C. is recommended.

Deposition pressures of 30 mN M⁻¹ and 35 mN M⁻¹ were satisfactory forR_(f) PyrBr, whereas 25 mN M⁻¹ led to lower surface density of thetransferred compound and 40 mN M⁻¹ apparently produced crowding anddisorientation of some molecules. Therefore, a pressure range of 30-35mN M⁻¹ is recommended.

The experimental data demonstrate that a single monomolecular layer,which is approximately 20 Å thick, is most effective in preventingfouling. Multiple layering, which would lead to greater materials costsand much higher processing costs, is also undersirable from thestandpoint of ultimate performance.

A dipping speed of 0.1 cm/sec for deposition of R_(f) PyrBr wasutilized. This speed was determined by observation of the meniscus,which is horizontal and smooth at appropriate transfer rates.

The category of liquid-liquid separation membrane types which can betreated by the present invention includes all those intended for theseparation of ions or ionic, colloidal, crystalline, particulate, orvaporizable material from liquids. In addition, the category of membranetypes is not limited to polymeric materials and includes membranesdesignated as electrodialysis, cation-exchange, anion-exchange, bipolar,reverse-osmosis, ultrafiltration, pervaporation, and hemodialysismembranes, but it does not exclude any selective membranes, known by anyother designation, intended to carry out a process that can be describedas the separation of ions or ionic, colloidal, crystalline, particulate,or vaporizable matter from liquids.

Experimental observations and data obtained during separation processesthat employed two types of anion-exchange membranes and two types ofreverse-osmosis membranes are set forth. The membranes were treated bydeposition of oriented layers of a fluorinated long-chain pyridiniumbromide. Also, comparable data are disclosed for control membranes thatare identical but untreated.

An alternative embodiment of the concept of the invention, with specificapplication to separatory membranes, is that oriented deposition ofappropriate long-chain amphiphilic compounds can be used to decrease thescaling tendencies of anion- and cation-exchange electrodialysismembranes. An electrodialysis membrane tends during operation to buildup regions of high pH at the surface that is not fouled by colloidalmaterials. As they come in contact with this region, many inorganiccations commonly present in water (calcium, magnesium, etc.) forminsoluble materials that precipitate as scale on the membrane surface.Accumulated scale, like layers of foulant, increases the electricalresistance and decreases the flux of the membrane.

Surface modification by deposition of an appropriately orientedmonolayer on any membrane face that is prone to scaling should preventthe attachment of materials such as inorganic oxides and hydroxides.Thus, although insoluble compounds may continue to form, they can haveno deleterious effect upon the operation of the membrane.

A further embodiment of the present invention is that orienteddeposition of appropriate long-chain amphiphilic compounds can be usedto enhance the inherent selectivities of several categories of selectivemembranes. The categories of selective membrane types include selectivesemipermeable membranes intended for the separation of ions or ionic,colloidal, crystalline, vaporizable or particulate matter from liquids(e.g., salt from brackish water or proteins from cheese whey); andselective semipermeable membranes intended for the separation of oneliquid from another liquid (e.g., ethanol and water).

It has been demonstrated, for example, that oriented deposited layers ofa nonfluorinated long-chain fatty acid will impart ethanol selectivityto a membrane that is intrinsically water-selective or increase theethanol selectivity of an ethanol-selective membrane. That confirms thehypothesis of the present invention that deposited oriented layers of anamphiphilic material exhibiting a strong affinity for one component of amixture will confer selectivity for that component upon a separatorymembrane.

The present invention may also be useful for the enhancement of the gasselectivities of gas-gas separation membranes (e.g., separating nitrogenor oxygen from the air). Colloidal fouling does not present difficultiesin gas-separation processes, but such processes are not presentlycost-effective because of the relatively low selectivities of availablemembranes.

Appropriate selection of amphiphilic materials may lead to thesimultaneous desirable modification of more than one property of amembrane. For example, deposited oriented layers of a selectedamphiphilic material might simultaneously heighten the water selectivityand reduce the fouling propensity and scaling tendency of a givenmembrane.

As another alternative embodiment closely related to the treatment ofthe surfaces of semipermeable membranes, the use of the presentinvention mitigates the fouling of anion- and cation-exchange resins,macroreticular resins, zeolites and similar materials intended for theseparation of one component from a mixture or solution. This mitigationwould be accomplished by deposition of appropriate oriented amphiphiliclayers on the surfaces of the resin particles, after their preparationunder nonfouling conditions and before their exposure to foulingconditions.

The concept of beneficial surface modification by deposition or orientedamphiphilic layers has several applications that are unrelated to themodification of semipermeable membranes. These include the following:modification of heat-transfer surfaces to promote dropwise condensationand to mitigate fouling, microbial growth, and scaling; modification ofthe surfaces of liners for solar-energy ponds to mitigate fouling andmicrobial growth; modification of marine surfaces to mitigate fouling,microbial growth, and inorganic scaling; modification of the surfaces ofmetals to mitigate their tendencies to corrode when exposed to certainenvironments; modification of the surfaces of photochemical solarconverters to protect them from oxide formation and to enhance theirlight absorption; modification of metal and polymer surfaces to heightenor reduce either their adhesive characteristics or their lubricities;modification of the surfaces of biomaterials used on prosthetic devices,bioimplants, etc. to minimize biorejection; modification of the surfacesof dialysis membranes to minimize both hemolysis and fouling; andmodification of the surfaces of dialysis membranes to increase theirselectivities for blood factors found to be associated with renallesions, rheumatoid arthritis, muscular dystrophy, schizophrenia, andother diseases.

BRIEF DESCRIPTION OF THE FIGURES OF DRAWINGS

FIGS. 1A-C are representative illustrations of the steps leading to thefouling of an anion-exchange membrane;

FIG. 2 is a series of schematic diagrams of the deposition of Blodgett"y-layers";

FIG. 3 is series of schematic diagrams of the deposition of Blodgett"x-layers";

FIG. 4 is a schematic diagram of the manner in which a close-packedmonolayer may bridge over surface roughness as it is deposited on asolid;

FIG. 5 is a schematic representation of the hydraulic circuit forfouling evaluations;

FIG. 6 is a representation of the electrical circuit for the foulingevaluation stack;

FIG. 7 is a schematic diagram of the assembled test electrodialysiscell;

FIG. 8 is a pressure-area curve for arachidic acid at 23.3° C.;

FIG. 9 is a π-A curve of R_(f) PyrBr;

FIGS. 10A-D are schematic representations of contact-angle decay atmembrane surfaces;

FIG. 11 is a contact angle-time surve for contact-angle decay onmembranes coated with three layers of R_(f) PyrBr at 25° C. and 25 mNM⁻¹ ;

FIG. 12 is a contact angle-time curve for contact-angle decay on threelayers of R_(f) PyrBr at 25° C. and 30 mN M⁻¹ ; and

FIG. 13 is a contact angle-time curve for contact-angle decay onmembranes coated with three layers (Sample 5) and ten layers (Sample 6)of R_(f) PyrBr at 10.5° C. and 40 mN M⁻¹.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

I. AN ANALYSIS OF CONCENTRATION POLARIZATION AND FOULING

A. Mechanism of Polarization and Fouling

The limiting current (i_(lim)) of an electrodialysis stack is generallyagreed to be that current density at which the boundary-layerconcentration of salt ions approaches zero. Current continues to passonly because water dissociates to form hydrogen and hydroxyl ions, aprocess that requires a high energy input. As a rule, industrialdesalination operations avoid this region of inefficiency by operatingat 70% of the limiting current, where the current-voltage relationshipis linear.

Some degree of concentration polarization, even at current densities farbelow the limiting current density, occurs during electrolysis with alltypes of ion-exchange membranes. Hydrogen and hydroxyl ions resultingfrom the "splitting" of water are involved in the conduction process,with hydrogen ions accumulating on the depleting surfaces of anion-exchange membrane while hydroxyl ions are transported through themembrane structure. Organic anions are converted, as they reach theresulting localized region of low pH, to sparingly soluble organicacids, which then deposit on a positively charged anion-exchangemembrane. Once this occurs, the bipolar membrane produced gives rise toeven faster production of hydrogen ions and the rate of deposition oforganic material (i.e., fouling) increases.

A basic separation unit comprises a cation-exchange and ananion-exchange membrane mounted between two electrodes, with electrolytesolution flowing through the enclosed compartments. The flow ensuresgood mixing in the center of the compartment, but its effect diminishesas the surfaces of the membranes are approached. In the static boundarylayers immediately in front of and behind the membranes, ions aretransported only by electrolytic transfer and diffusion; in the mixedzone, ion transport is a function of electrolytic transfer, diffusion,and physical mixing.

With the passage of an electrolytic current through the system, anionsmigrate toward the anode and cations transfer toward the cathode. If theelectrolyte is KCl, cations and anions share equally in the passage ofcurrent through the solution bulk, and the transference number is 0.50for both. The membranes, however, are selective; the transference numberfor potassium is essentially 1.0 in the cation-exchange membrane and 0.0in the anion-exchange membrane. Similarly, the transference number forchloride ion is 1.0 in the anion-exchange membrane and 0.0 in thecation-exchange membrane. Chloride ions carry only 50% of the electricalcurrent in solution, but 100% of the current through the anion-exchangemembrane. These differences in transference numbers between solution andmembrane are the source of the depletion and concentration effects thatmake electrodialysis a valuable separation procedure. The sametransference-number differences also lead to difficulties likeconcentration polarization.

If one faraday of electricity is passed through the above-describedelectrodialysis cell, 0.5 g eq of chloride will be transferred to oraway from the membrane surface, and 1.0 g eq of chloride will betransferred through the membrane. There will be a concentration ofchloride ions at the rear surface of the anion-exchange membrane, but adepletion of chloride ions at its front surface. At steady state,chloride ions that are not electrolytically transported to the frontsurface must be supplied by diffusion through the static boundarylayers, and concentration gradients are established. This steady-statecondition can be described by Equation 1: ##EQU1## where: i=currentdensity, coulomb sec⁻¹ cm⁻²

F=faraday, 96,500 coulomb eq⁻¹

t_(s) ⁻ =transference number of anion in solution

D=diffusion coefficient of ion

C_(b) =concentration of ion in the bulk

C₁ =concentration of ion in the boundary layer

δ=thickness of boundary layer

t_(m) ⁻ =transference number of anion in membrane

This equation can be rearranged to Equation 2: ##EQU2## which showsthat, as current density increases, the boundary-layer anionconcentration approaches zero. Although some hydroxyl ions aretransported through the membrane and some hydrogen ions accumulate atthe membrane surface even at very low current densities, the fraction ofcurrent carried by hydroxyl ions is insignificant until thelimiting-current density is reached. At this point, continued passage ofcurrent requires that hydroxyl ions take the place of theno-longer-available anions, and hydroxyl ions can be furnished only bythe ionization of water. At higher current densities, therefore,hydrogen and hydroxyl ion concentrations become larger relative to theconcentrations of other ions. Hydroxyl ions are transferred through theanion-exchange membrane, leaving an excess of hydrogen ions on itssurface (FIG. 1A), which markedly lowers the pH at that surface.

Most naturally occurring organic contaminants bear negative charges, andtheir acid forms are insoluble. When the supply of hydrogen ions at thedepleting surface of an anion-exchange membrane is sufficiently great,colloidal organic substances are partially neutralized (FIG. 1B) andprecipitation is initiated (FIG. 1C). The autocatalytic nature of thiscolloidal fouling, its energetic consequences, and some of the membranecharacteristics contributing to it can be summarized as follows:

Whenever current is passed, salt ions are depleted near membranesurfaces because of transference number differences between solutionsand membrane, and concentration polarization occurs.

Further passage of current requires the furnishing of hydrogen andhydroxyl ions through the continuous ionization of water.

Hydrogen ions accumulate at the depleting surfaces of anion-exchangemembranes whenever concentration polarization causes water molecules toionize.

Organic anions are driven toward the surfaces of anion-exchangemembranes by the electric field.

In the zone in which hydrogen ions accumulate, organic anions areconverted to sparingly soluble acids, which deposit on the membrane.

Precipitation of negatively charged material at the surface of amembrane bearing fixed positive charges effectively produces a "bilayermembrane". At the interface of the oppositely charged ion-exchangematerials, negative ions migrate through the anion-exchange membrane andpositive ions migrate through the negatively charged colloidal layers.The net result is salt depletion in the interfacial region, whichfurther potentiates water ionization, hydrogen ion production, andcolloidal precipitation (FIG. 1C).

Once deposition of organic material occurs, the deposited material istightly held by van der Waals forces and is difficult to completelyremove.

The energy required for continuous ionization of the water that diffusesto the interfacial region leads to an increase in the apparent membraneresistance.

The resistance of the interfacial layer of solution also becomes higheras it is depleted of electrolyte, raising the total resistance of thesystem.

The fouling behaviors of a given membrane type and composition can varywidely between samples; "glossy" surfaces appear to be related tofouling resistance. Grossman, G. and Sonin, A., Office of Saline WaterResearch and Development Progress Report, 742 (1971).

The degree of microscopic surface homogeneity is important. Membranescontaining reinforcing materials, or membranes with micro-heterogeneoussurfaces show a greater tendency to foul rapidly. Korngold, E.; deKorosy, F.; Rahav, R.; and Taboch, M., Desalination 8 (1970), 195.

The tendency to foul is the same in electrodialysis stacks containingonly anion-exchange membranes as in those comprised of alternatingcation-exchange and anion-exchange membranes.

Anion-exchange membranes have much greater fouling propensities thancation-exchange membranes, because most organic contaminants arenegatively charged.

These observations cumulatively support the hypothesis that membranepolarization leading to fouling is primarily a surface-controlledphenomenon. Therefore, a means of permanently modifying an operatingmembrane surface without changing the bulk electrical properties offersthe best hope of producing desalination membranes with long operatinglifetimes.

B. A Mathematical Model for Fouling

Adopting the physical model of fouling that evolved from the work ofCooke and Korngold, et al., Electrochem. Acta 4 (1960), 1979; andDesalination 8 (1970), 195, Grossman and Sonin derived an expression forthe amount of fouling in terms of the resulting reduction in limitingcurrent. Office of Saline Water Research and Development Progress Report813 (1972); Desalination 10 (1972), 157; Desalination 12 (1973), 107.They concluded that a fouling film with the same charge as the substratewould not affect the limiting current. An oppositely charged, extremelythin film can cause marked limiting-current reduction, the thinnessrequired for effective fouling being an inverse function of theconcentration of fixed charge. A neutral film can reduce the limitingcurrent but, to do so, it must be many times thicker than an oppositelycharged film.

When a Blodgett layer is deposited on a substrate, its thickness,surface concentration, and charge density are both known andcontrollable. This fact can provide an excellent basis for detailedexperimental testing of the Grossman-Sonin model for membranes fouled byknown thicknesses of oppositely charged or neutral molecules.

II. BLODGETT MULTILAYERS

A. The Monolayer Assembly Technique

If a solution of amphiphilic molecules in a hydrocarbon solvent isgently dropped on a water surface, the drops will fan out as the solventevaporates. Simultaneously, the amphiphilic molecules become orientedinto a solid monolayer exactly one molecule thick, with their polar"heads" at the water interface and their non-polar "tails" at the airinterface. Langmuir, I., Science 87 (1938), 493. This monolayer lowersthe surface tension of the water by an amount equal to its own "surfacepressure".

At a given surface pressure, each type of monolayer film occupies acharacteristic surface area. If the molecules are pushed together by amoving barrier, regions characterized by different compressibilitiesappear until the film "collapses", usually at around 20 Å² /molecule forfatty acid monolayers.

If a monomolecular film is in the so-called "condensed" region thatcorresponds to high surface pressures, it will readily transfer to asolid that is passed vertically through it. Blodgett, K. B., J. Am.Chem. Soc., 57 (1935), 1007. The Blodgett-Kuhn dipping apparatusprovides a moveable polyethylene float activated by a suspended weight,so that constant surface pressure and constant molecular area of thespread film enclosed by the barrier are maintained throughout thedipping procedure. As a monolayer of film transfers from the liquidsurface to the solid, the floating barrier moves forward so that thearea of the spread film decreases by the area of both sides of thedipped solid. The "transfer ratio", i.e., the areal coverage on thesolid relative to the areal coverage on the water is virtually unity foreach layer, and orientation is conserved during transfer.

Repetitive dipping of the solid to be treated results in the pickup ofsuccessive monolayers, oriented in a y (head-head, tail-tail) (FIG. 2)or an x (head-tail, head-tail) (FIG. 3) pattern. This technique resultsin multilayers of molecules oriented perpendicular to the surface onwhich they are deposited. The number of deposited layers is found byactual counting of the number of forward movements of the barrier duringsuccessive dips. Layering patterns, as well as the total number oflayers that can be deposited, are dictated by the chemical and stericnature of the amphiphilic molecules. After a given number of layers hasbeen attached, an assembly becomes "autophobic", rejecting the additionof more layers of any substance, including itself.

Whether or not a substance will transfer as a multilayer to a solidsubstrate depends on several factors, including the attraction of themolecules for the water surface, the cohesive attraction between themolecules and the attraction of the molecules to the solid substrate.

B. The Properties of Monolayer Assemblies

Monolayers attached in the proposed manner have unique properties fortwo reasons: each layer is reproducibly one molecule thick; and thepolar-nonpolar portions of the molecules in each layer are strictlyoriented with regard to the substrate. These features are found in themembranes of biological cells and are believed to be critical to naturalphenomena like activated transport.

Oriented monolayers adhere to their substrates with extraordinarystrength. In the case of a stearate film on ordinary glass, for example,penetration of the carboxylate groups of the fatty acid salt into theglass surface is so extensive that the salt can be removed only bysandblasting, which, of course, also destroys the involved region of thesubstrate. Some substances can form monolayer assemblies up to 4,000layers thick and many of these assemblies, also called Blodgettmultilayers, are indefinitely stable. For example, the nonreflectingglass used in optical equipment is prepared by monolayer depositiontechniques.

Although assembled monolayers are stable, they are also "penetrable", aterm suggested by Sobotka to emphasize that passage through multilayersis possible because they are in continuous thermal motion. J. ColloidSci. 11 (1956), 435.

The application of monolayers to the surfaces of membranes should havelittle influence on their ion-exchange properties, even if themonolayers are charged. This is true because of the exceedingly smallnumber of charged groups in even a completely ionized monolayer. The vander Waals' forces responsible for hydrophilicity decay with the sixthpower of the distance and therefore operate over an exceedingly smallrange, whereas ion-exchange depends on electrostatic forces, whichdecreases only with the square of the distance. Thus, depositedmultilayers will probably allow sufficiently close approach of ions sothat they can be attracted to the fixed charge groups on the membrane.

1. The alteration of surface hydrophilicity by attachment of monolayerassemblies

Langmuir was the first to observe that a metal surface coated bystearate multilayers became non-wettable by water and also by manyhydrocarbons. J. Franklin Inst. 218 (1934), 143. The coated surface hadbecome both hydrophobic and oleophobic. Orientation of the attachedmultilayers had apparently produced a smooth surface consisting ofclosely packed and strictly aligned methyl groups, i.e., a very"low-energy" surface. Wettability of a solid is thus dependent only uponits outermost atomic layers. Shafrin, E. G. and Zisman, W. A., J. Phys.Chem. 64 (1960), 519. In general, factors that increase the polarity ofa surface, including unsaturation of hydrocarbon groups, increasewettability. Hydrogenation decreases wettability, and fluorination has astill more marked effect.

Zisman found that surfaces coated by multilayers of perfluorolauric acidexhibited the lowest surface energy of any surface yet prepared.Shulman, F. and Zisman, W. A., J. Colloid Sci. 7 (1952), 465. Thesesurfaces are both hydrophobic and oleophobic, repelling even alkaneswith great efficiency.

The implications are that an appropriate multilayer coating willeliminate the water wettability of a membrane surface by increasing itshydrophobicity, and it will simultaneously reduce adhesive attractionsbetween the membrane surface and organic materials in the raw waters byincreasing the surface oleophobicity.

2. Reduction of surface heterogeneity by deposited multilayers

It has been found that crevices and pores in a surface give rise tovariabilities in wetting and de-wetting behavior, implying that thewettabilities of rough surfaces and smooth surfaces are very different.Bikerman demonstrated the integrity of deposited multilayer films byattaching films of barium stearate to wire gauze with apertures about0.53-mm across. Proc. Roy. Soc. A. 170 (1939), 130. The transfer ratiobased on the gross area of the gauze was nearly unity, exactly the sameas it would have been if the object coated had been a solid withoutholes or subdivisions. Thus, the oriented layers attached themselves tothe gauze at the available points and possessed sufficient lateralcohesiveness to bridge the relatively large intervening gaps as long asthey were kept wet. They thereby conferred a homogeneous character upona highly heterogeneous surface. (See FIG. 4).

Day and Ringsdorf have carried out experiments in which diacetylenemonocarbonic acids were polymerized at a water surface and transferredas bilayers to porous substrates. J. Polym. Sci. Polym. Lett. 16 (1978),205. These coatings were 60-Å thick and could be made to bridge pores upto 0.5 mm in diameter on the solid substrates. These experimentsdemonstrate that deposited multilayers can lend a homogeneous charactereven to solids with gross heterogeneities. The effect of successivemonolayers upo the fouling propensities of a desalination membrane witha roughened surface should give a ready evaluation of the importance ofroughness and heterogeneity.

3. Conditioning of Langmuir-Blodgett multilayers

The properties of deposited monolayer assemblies can be altered to fitspecific needs. For instance, a hydrophobic film can be renderedhydrophilic by post-treatment with dilute solutions of polyvalentcations like thorium nitrate. If the assemblies are mixtures, either ofa monomer and its polymers, or of an acid and its salt, one componentcan be removed by a suitable solvent, leaving a "skeletonized" film ofregular structure. The degree of polymerization, or the initial relativeconcentrations of soap and acid, provide methods of controlling thefinal coverage on the substrate.

The "depth" of coverage at the membrane surface can be controlledthrough the number of monolayers applied. The percentage of surface thatis covered to this depth can be determined by the proportions of, forexample, polymerizable and nonpolymerizable monomers in the depositedlayers. Fort et al. showed that post-polymerization at the surface,followed by solvent leaching, effectively removed ethyl stearate frompolyvinyl stearate. J. Coll. Interface Sci. 47 (1974), 705. Theattachment of polyvinyl stearate to the membrane and the structuralintegrity of the polymer remained unchanged. The apertures of theseskeletonized films can be "refilled" by water or hydrocarbons. Sobotka,H., Proceedings of the Conference on Biochemical Problems of Lipids,Butterworths, London, 1956, p. 108.

4. Gas permeation through membranes modified by monolayer deposition

Two types of membrane modifications by monolayer deposition have beencarried out by Quinn. Science 159 (1968), 636; J. Coll. Interf. Sci. 27(1968), 193; and Biophys. J. 12 (1972), 990. In one, severalgas-permeable membranes were coated with assembled monolayers, and theeffects on the permeation rates of different gases were evaluated asfunctions of the nature and number of the multilayers. Permeabilities ofthe substrate polymers were markedly decreased by attachment of stearicacid or 3β-cholestanol, but the effect of oleic acid was much smaller.Presumably, steric influences on the packing of the multilayers canexplain these differences. These workers also studied the effect of poresizes on gas permeation rates, modifying the reproducibly-sized pores oftrack-etched mica membranes by depositing stearate multilayers. As thelayers dried, they migrated into the pores by surface diffusion. Poreradii determined by Knudsen gas flow showed excellent correlation withthe modified radii predicted from the number of deposited multilayers.

5. Blodgett deposition on reverse-osmosis membranes

Langmuir-Blodgett layers have been deposited and polymerized on porouspolysulfone backing materials to produce asymmetric reverse-osmosismembranes. Fort, T., Jr. and Lando, J., Office of Saline Water Researchand Development Progress Report, 74-944 (1974). High salt-rejectionsamples could be prepared with coatings comprising 18 layers ofcellulose acetate, but many technical difficulties were encounteredduring the deposition procedures, and cracks leading to leakage throughthe multilayers were frequent.

Such defects would be less deleterious to the successful utilization ofthis invention than to the process described by Fort and Lando.Amphiphilic molecule deposition was designed in this invention tomoderate the polarization and occlusion tendencies of working membranes,whereas the amphiphilic molecule layers that they deposited on poroussupports were intended to become the working parts of reverse-osmosismembranes. Any defects thus led to losses of basic function, whiledefects in the coatings of the present invention would lead only to alower percentage of modification of undesirable properties. As discussedinfra, Langmuir-Blodgett layers may also be used to enhance the foulingresistance of standard reverse-osmosis membranes.

6. "Electrets" as fouling preventives in reverse-osmosis experiments

Wallace and Gable compared fouling behavior of unmodified celluloseacetate reverse osmosis membranes with that of identical membranes thathad been made into "electrets", Polym. Eng. and Sci. 14, (1974), 92.These are essentially solid-phase condensers, with negative chargesaligned along the "skinned" surface, and are electroformed by chargingin a five-layer capacitor. Low-humidity measurements of the net surfacecharge showed a rapid decay rate during the first 24 hours, after whichdetectable charge persisted for more than 70 days, the total span ofobservation. Immersion in distilled water after 20 days brought aboutextremely rapid charge dissipation.

When electret membranes were used in reverse osmosis systems, both theamount and the adherence of foulant deposits were reduced. In additionto repelling colloidal tannic acid, the electrets absorbed lesscolloidal iron oxide. Salt rejection was unchanged.

No data were given on decay behavior of in-service electret membranes oron the length of time between the "electroforming" and initiation of thereverse osmosis testing described. It is logical to assume thatelectrets would decay rapidly in water that contains ions, sincerandomization of the aligned polar portions of the cellulose acetatewill be encouraged by a randomly charged environment and by waterpermeation. However, it is apparent that the presence of negative chargeon the electret surface minimized difficulties with fouling, even thoughthis charge was weak and shortlived.

Deposition of assembled monolayers to form a sheath may produce the sameprotective effect as "electret" production, with the additionaladvantage of long-term stability.

7. Electrodeposited polyelectrolytes on cation-exchange membranes

Sata and Mizutani have reported treatments of commercial cation-exchangemembranes by surface coatings of various cationic polyelectrolytes. J.Polym. Sci. Polym. Chem. Ed. 17 (1979), 1199. The polyelectrolytes wereapplied either by electrodeposition or by adsorption from solution, andwould therefore not exhibit the strict molecular orientation andlayering behavior of Blodgett layers. However, the properties ofelectrodeposited layers may approach those of monolayer assemblies, thusindicating the direction and degree of modification that can beexpected. The coatings affected current efficiencies, electricalresistances and selectivities between univalent and divalent cations.

In all cases, the electrodeposited layers produced greater changes inmembrane properties, and were more compact and thicker than adsorbedlayers. The electrodeposited layers effectively prevented fouling byionic surface-active agents, so that the membrane resistance remainedconstant during electrodialysis of solutions of these agents.

Monolayer assembling should confer the same tenacity of attachment thatelectrodeposition did in Sata's work, with the added benefits ofmolecular orientation and the ability to use minimal, preciselycontrolled, coating thicknesses. The anti-fouling effect should be thesame in a desalination environment as in a polyelectrolyte dialysissystem.

III. SPECIAL APPARATUS

A. The Wilhemy Balance for Surface Pressure-Area Measurements

An apparatus was constructed for measuring the surface pressure (π) ofan oriented monomolecular film on water as a function of availablemolecular area (A). The Wilhemy balance comprises a shallow film troughof solid Teflon on a heavy aluminum base equipped with leveling feet. Astainless-steel rod, piercing a gasket at one end of the enclosingLucite box, controls molecular area by manipulation of a spring-loadedTeflon barrier straddling the trough. Two-dimensional pressure changesare calculated from differences in the apparent weight of a 3-cm squareof Schleicher and Schull No. 589 filter paper. This piece of paper (a"Wilhemy plate", chosen because it is completely wetted and nocontact-angle correction need be included in calculations) hangs by asilver chain from the beam arm of a modified Troemner Model 5100specific-gravity balance. The observed surface pressure, π, isequivalent to the change in surface tension due to the monolayer filmand is found from Equation 3: ##EQU3## here g=gravitationalconstant=980.7 cm sec⁻²

ΔG=change in apparent weight of "plate" relative to weight in waterwithout a monolayer

w=width of plate=3.0 cm

t=thickness of plate=0.005 cm

For our system, the constants can be lumped together to give Equation 4:

    π=-163.16 (ΔG)mN M.sup.-1                         (4)

The validities of our measured values for surface pressure and area werechecked by reproducing the well-known curve for arachidic acid. (SeeFIG. 8).

B. Blodgett-Kuhn Dipping Trough

A dipping trough was constructed so that the water or salt hypophase(liquid supporting the monolayer) and any monomolecular film spread uponit are in contact only with thoroughly cleaned Teflon or Pyrex glass.The windlass is machined to move clamped membranes smoothly up and downduring deposition, and is hand-controlled for individual monitoring ofeach trip.

The polyethylene float confirming the spread monolayer is free to moveforward within the confines of parallel Teflon bars. Its position at agiven moment reflects a balance between the surface pressure of thespread film and the force exerted by an aluminum weight that hangsfreely from the front of the apparatus and is attached to the float by anylon thread. This apparatus is enclosed by a protective Lucite box. Inshakedown runs, arachidic acid multilayers were deposited on glassslides, for comparison with literature reports.

C. Contact-Angle Apparatus

A highly sophisticated contact-angle goniometer (Rame-Hart, Inc., Model100-00) was modified and refined from a design originated at the NavalResearch Laboratories. A microsyringe is used to deliver calibrateddrops of liquid to the surface being evaluated. The apparatus is mountedon the trunnions of a tilting base so that the alignment of optics andspecimen is held constant during measurement of advancing and recedingangles. Special film clamps are used to secure membrane strips flat onthe specimen stage. The entire apparatus is enclosed in a protectiveLucite box.

D. The Laboratory Stack for Fouling Evaluations

1. Stack construction

a. The separators

Three-inch square separators were designed to hold twelve membranesrigid in each electrodialysis cell. They were built individually onwooden frames, with Lucite side pieces and evenly spaced Tygon"spaghetti" tubing potted into Silastic cement serving as end pieces.After the Silastic cured, the separators were removed from the frames,and the Tygon tubes were severed at the inside surfaces of the endpieces. Their other ends were potted into large-diameter acrylic tubesfor attachment to the hydraulic system. Entry and exit of salt solutionsthrough the resulting multiple ports ensured thorough, well-distributedflushing of all membrane surfaces, with a good approximation to laminarflow.

b. The hydraulic circuit

As FIG. 5 illustrates, streams of solution circulate from separatereservoirs through the electrodialysis cell. Potassium chloride is theelectrolyte of choice in the test compartments, potassium acetate (KOAC)is the electrolyte for the electrode compartments. A Cole-Parmer ModelWZ1R057 Masterflex pump with one add-on head drives both solutionsthrough silicone rubber tubing at flow rates up to 2 L/min. The valvesof the system are adjusted during operation to equalize the flow betweentest compartments and electrode compartments.

c. The electrical circuits

An Epsco Model D-612T power supply establishes selected potentialsbetween a platinized-titanium anode and a stainless-steel cathode, whichare sealed into the two Micarta end blocks that forms the ends of theelectrodialysis cell. Although the power supply has readouts for bothvoltage and amperage, a milliammeter and a voltmeter were included inthe circuit for additional precision (FIG. 6).

d. Stack assembly

The cell is clamped together as diagrammed in FIG. 7.

e. Shakedown fouling runs

The fouling test stack was assembled with eight test membranes in thecentral positions, treated sides facing the cathode. Untreated AMF A-63anion-exchange membranes were used as electrode membranes and asisolating membranes between the KCl and KOAc streams.

Potassium acetate (KOAc) was selected as the electrolyte for theelectrode compartments to prevent chlorine evolution, which mightobscure or otherwise interfere with the fouling process. Potassiumchloride (KCl) was used as the electrolyte in the test compartmentsbecause potassium and chloride ions have approximately equaltransference numbers (0.50) in aqueous solution. The test solution alsocontained 0.1% of sodium humate (Aldrich Chemical Company, Milwaukee,Wisc.), which makes it more concentrated than natural waters by a factorof about 10⁴.

All test membranes and isolating membranes were equilibrated in KClsolution prior to insertion in the cell, and the twoelectrode-compartment membranes were equilibrated in KOAc. They werearranged in the cell in order, with Sample 1 in the cathode compartment,Number 2 isolating the cathode compartment from the test compartment,Numbers 3 through 6 in the test compartments, Number 7 separating theanode and test compartments, and Number 8 in the anode compartment (seeFIG. 7). In cases where the test membranes were coated on only one side,the treated side faced the cathode. Three 13.5-mil gaskets between themembranes and separators have good sealing with free solution flow.

IV. EXPERIMENTAL DETAILS

A. Chemicals

1. Chemicals for fouling tests

Sodium humate (technical grade) was purchased from Aldrich Chemical Co.,Milwaukee, Wisc.

2. Chemicals for Blodgett deposition

Samples of surface-active compounds bearing perfluorinated carbons atthe end of their chains opposite to various functional groups werefurnished by the Commercial Chemicals Division/3M Center, St. Paul,Minn. These compounds are laboratory prototypes, and 3M policy precludesrevealing their molecular weights or any information other than thatshown in Table I, below.

Other fluorinated, non-fluorinated and polymerizable compounds werepurchased from commercial suppliers. Although it is not certain that allof these are sufficiently surface-active to properly undergo orienteddeposition, fluorinated molecules are usually surface active at a muchlower molecular weight or shorter chain length than their hydrogenatedhomologues.

                  TABLE I                                                         ______________________________________                                        CHEMICALS FOR BLODGETT DEPOSITION                                                                        Molec-                                             Compound Name              ular     Catalog                                   or Formula      Source     Weight   No.                                       ______________________________________                                        Fluorinated Compounds                                                         R.sub.fCOOH     3M                  L-1058                                    R.sub.fSO.sub.2 K.sup.+                                                                       3M                  L-1159                                    R.sub.f NHMe    3M                  L-2338                                    R.sub.fPO (OH).sub.2                                                                          3M                  L-4317                                     ##STR1##       3M                  L-4745                                    Hexadecalfluoro-                                                                              Gallard-   432      F-4530                                    1-nonanol       Schlesinger                                                   Perfluorotributylamine                                                                        Gallard-   671      F-6220                                                    Schlesinger                                                   Perfluorodecanoic acid                                                                        PCR        514      10614-6                                   11-H-Eicosafluorounde-                                                                        PCR        546      13174-8                                   canoic acid                                                                   Non-fluorinated compounds                                                     Cetylpyridinium bromide                                                                       Sigma      384      C5881                                     Hexadecyltrimethyl-                                                                           Sigma      364      H5882                                     ammonium bromide                                                              Dodecyltetramethyl-                                                                           Sigma      308      D8638                                     ammonium bromide                                                              Tetradectyltrimethyl-                                                                         Sigma      336      T4762                                     ammonium bromide                                                              Polymerizable monomers                                                        Hexafluoroisopropyl                                                                           Polysciences                                                                             236      2401                                      methacrylate                                                                  Hexafluoroispropyl                                                                            Polysciences                                                                             222      2400                                      acrylate                                                                      ______________________________________                                    

B. Anion-exchange Membranes

1. AMF A-63

The anion-exchange membrane, AMF A-63, is lightly crosslinkedpolystyrene imbibed into polyethylene film, chlorinated, and quaternizedwith dimethylethanolamine. Korngold focused most of the experiments inhis detailed study of fouling of anionselective membranes on thismaterial, providing extensive data on fouling of A-63 as a function oftime, current density, salt concentration, feed solution velocity andbuffered pH. Desalination 8 (1972), 195.

A-63 is not commercially available at present, but Dr. Richard N. Smithof Southern Research Institute, Birmingham, Ala., kindly donated a largesupply, which he personally prepared while employed by AMF Corporation.

2. SORI A568-007

Kressman and Tye suggested many years ago that membranes exposed to tapwater during their manufacture were effectively pre-fouled before anyexposure to the colloidal content of natural waters. J. Electrochem.Soc. 116 (1969), 25. Therefore, the critical initiating step thatcatalyzes fouling had already taken place.

For this reason, and because there was difficulty in evaluating limitingcurrents to characterize the AMF A-63 membranes, novel, low-resistanceanion-exchange membranes were prepared under carefully controllednon-fouling conditions. No water is used during actual preparation, andthe membranes were exposed only to reagent-grade chemicals, withsolutions in Milli-Q water used for equilibration to saline conditions.Because oils and surfactants are omnipresent on skin, gloves were wornduring the handling, and the samples were protected from other sourcesof contamination.

C. Pressure-Area Curves of Selected Compounds

A fluorinated aromatic heterocyclic bromide

Initially, a 1:1 chloroform-methanol mixture was used as a spreadingsolvent for R_(f) PyrBr, a fluorinated pyridinium bromide furnished byDr. Kenneth D. Goebel of the 3M Corporation. This compound is alaboratory prototype, and its molecular weight and precise compositionare proprietary. It is believed to be a long hydrocarbon chain whichlinks the pyridinium bromide group with a fluorinated end group.

Because of its compatibility with water, 1:1 chloroform methanol is notan ideal spreading solvent, and thirteen possible alternatives werescreened. This study showed that the first attempts at dissolving andspreading R_(f) PyrBr had pinpointed an optimal solvent; in fact, thiscompound was unable to be dissolved in chloroform-methanol mixturescontaining less than 50% methanol. With sufficient care, this systemproduces reliable monolayers, especially at reduced temperatures, asshown by the π-A curves in FIG. 9. Because the molecular weight is notknown, units of Å² /μg×10¹⁶ were used instead of Å² /molecule for theseplots.

The behavior in high pressure ranges described by these curves is verydifferent from that observed for non-fluorinated compounds. Quiteordinary compressibility changes up to a surface pressure of 20 mN M⁻¹were observed. Most monolayers exhibit sharply decreasedcompressibilities above this pressure, causing the curve to becomenearly vertical until the collapse pressure is reached between 30 and 50mN M⁻¹. (See FIG. 8 for arachidic acid). In contrast, R_(f) PyrBrmonolayers are highly compressible up to 35 mN M⁻¹, with smoothtransitions between several compressibility ranges. At 55 mN M⁻¹, thefilm does not collapse, but it exhibits a constant surface pressure.Inducement of collapse in R_(f) PyrBr monolayers was never successful.

A definitive interpretation of this behavior is impossible withoutknowledge of the area available to each molecule at given surfacepressures. Nevertheless, the observed high compressibility at surfacepressures above 30 mN M⁻¹ would correspond with Gaines' statement that ". . . the fluorinated compounds occupy considerably larger areas inmonolayers on water than their hydrocarbon analogs . . . " InsolubleMonolayers at Liquid-Gas Interfaces, Interscience, New York, 1966. Thereis an impliction that, at least in such an oriented states, strongrepulsions exist between the fluorinated molecules. Increases of surfacepressure would be utilized in overcoming these repulsions, up to thepoint where solution in the water hypophase becomes energeticallypreferable to further compression or collapse. Thus, at 56 mN M⁻¹ ofsurface pressure, R_(f) PyrBr may be dissolving at a rate exactlybalanced by the rate of film compression.

Two facts critical to attaining the objectives of this research emergedfrom examination of the π-A plots for R_(f) PyrBr (FIG. 9). First, at 30mN M⁻¹, the highest deposition pressure heretofore used, this filmcannot be considered to be highly condensed. For effective film transferto a substrate, with a transfer ratio close to 1.00 and rigidorientation throughout the film, the monolayer must be compressed quiteclose to the collapse point.

Second, as suspected, R_(f) PyrBr has a nontrivial solubility in water.It is apparent from the parallel but offset π-A curves at differenttemperatures that the solubility is highly temperature dependent. Todeposit R_(f) PyrBr films of optimal compactness and coherence, the workmust be done at low temperatures and surface pressures of at least 35 mNM⁻¹.

D. Multilayer Deposition in the Blodgett-Kuhn Trough

1. Sample preparation

a. Membrane cleaning

A procedure was devised to free the surfaces of AMF A-63 anion-exchangemembrane from grease, surfactants, and other contaminants. Although itis probable that commercial anion-exchange membranes are somewhat fouledduring the manufacturing process itself, samples carefully cleansed ofremovable materials gave the most reliable baseline for evaluations.

In every step of the cleaning protocol, the operator wore clean gloves,and only Milli-Q water was allowed to contact the membrane samples. Useof this high-purity water, which is extremely low in both salt andorganic content, prevented further contamination and reversed, ifpossible, prior contamination.

The treatment included the following steps: brushscrub both sides ofeach membrane with a Milli-Q solution of Oxford Laboratory Cleaner;rinse in hot Milli-Q water; treat overnight with Milli-Q water at 70° C.in an ultrasonic cleaner; dry in an oven at 50° C.; and store in adesiccator.

b. Coding of samples

Completed samples are marked by a hole puncher with a code designatingwhich side is treated, the type of treatment, and the position of themembrane during dipping. It is then possible to differentiate, forexample, between the sample that faced the left front of the dippingtrough and the one that faced the right rear. Such differences inposition may lead to significant variations in properties, as observedby Fort and Lando for multilayered reverse-osmosis membranes. Office ofSaline Water Research and Development Progress Report 74-944 (1974).

c. Multilayer preparation

(1) Arachidic acid multilayers

Arachidic acid monolayers at 25 mN M⁻¹ and 25° C. were spread on Milli-Qwater from a 10⁻⁴ M chloroform solution. Float movements during membranedipping cycles were reproducible for clean, flat membranes. Sets ofmembranes coated with 1, 2, 3 and 10 y-layers (head-to-head,tail-to-tail) of arachidic acid were prepared. These samples providedsufficient material for a fouling-evaluation run as well as forexamination of wetting behavior, microscopic surface structure,electrical resistance, and transference number. Immediately afterpreparation, the samples were immersed in Milli-Q water and stored inwater until evaluation.

(2) Fluorinated pyridinium bromide multilayers

R_(f) PyrBr proved to be almost insoluble in chloroform, which is apreferred spreading solvent. It was highly soluble in methanol and wasspread from a 1:1 methanol-chloroform solution, the highest chloroformconcentration in which it would dissolve. This mixture is far toocompatible with water to be a good spreading solvent, and the utmostcare was required to prevent drops of solution from piercing the watersurface and dissolving in the hypophase. Although monolayers wereformed, it is probable that the pyridinium bromide was also able to swimout of the bulk hypophase onto the exposed water surface at the backedge of the float. This can create a competitive lowering of the surfacetension at that edge, and the force exerted on the enclosed film may beerratic.

Pressure-area curves indicated that a low temperature is desirable toensure formation of stable films and minimize solution of R_(f) PyrBr.By packing the dipping trough in an ice-brine slurry, its temperaturewas maintained between 9.0° and 11.5° C. Films were spread from a 10⁻⁴ Msolution in 1:1 chloroform-methanol.

On the basis of the π-A curves for R_(f) PyrBr, it was also decided towork at higher surface pressures than those used in the exploratoryphase. Sets of membranes were prepared coated with 1, 2, 3, and 10layers of R_(f) PyrBr, at deposition pressures of 30 mN M⁻¹ and 40 mNM⁻¹.

The differences in float behavior between the 30-mN M⁻¹ and 40-mN M⁻¹dipping runs were striking. At 30 mN M⁻¹, float movement (by which theamount of material deposited is measured) was always somewhat erraticduring immersion. Although the first several layers of R_(f) PyrBrtransferred as x-layers (head-tail-head . . . ), float movement on thethird to fifth emersion signalled the deposition of a y-layer(tail-tail-head-head . . . ). Additional y-layers were deposited atrandom between x-layer sequences on the 10-layer samples. Considerablerandomness thus occurred in the multilayer structures deposited at 30 mNM⁻¹, although examination by SEM (to be discussed later) portrays aremarkably coherent, frictionless and homogeneous surface.

On initiating R_(f) PyrBr depositions at 40 mN M⁻¹, it was decided to"vacuum" remaining monolayer from the hypophase surface between eachimmersion and emersion. This procedure ensures x-layering throughout theentire structure and eliminates any randomness in the multilayerpattern. All emersions under these conditions produced very clean,smooth minisci at the emerging membrane surfaces. This is a goodindicator that the surface is sub-microscopically smooth. Membranesamples dipped singly, so that both sides were coated with pyridiniumbromide, swelled and buckled to some extent, but not as severely assimilar samples dipped through arachidic acid. The buckling problem wasvirtually resolved when samples were sealed together for asymmetriccoating. All of the asymmetrically coated samples curled toward theircoated sides after separation from the sandwiches and drying, implyingthat the two sides are indeed different.

Through careful monitoring of the behavior of the meniscus at thevertical membrane surface, it was found that maximum deposition wasachieved when the rate of dipping was adjusted to ensure a smoothmeniscus at all times. For several emersions, maintaining a smooth,intact meniscus required a withdrawal rate as slow at 0.13 cm/min.

Another source of randomness was eliminated by the increase in surfacepressure. Float movements on emersion were about 40% larger than at 30mN M⁻¹, and they were very reproducible. Thus, more material wasdeposited with each layer, and the amount was identical from layer tolayer at the higher pressure. Because the molecular weight of R_(f)PyrBr is not known, the surface coverage cannot be estimated. It isprobable, however, that by transferring the monolayer films at surfacepressures about 30 mN M⁻¹, still greater film coherence would beachieved, which should give rise to a distinctive increase inhydrophobicity.

Immediately after preparation, all samples were immersed in Milli-Qwater and stored in water until evaluated.

E. Contact Angle Determination

All ion-exchange membranes sorb water, making true measurement of thecontact angle displayed by a sessile drop a practical impossibility.Soon after application, the profile of a standing water drop becomeslower while the membrane surface undergoes a simultaneous localizedrise. The phenomena responsible for the resulting continuous changes inbaseline and profile include at least the following: localizedabsorption of the water; diffusion of water into surrounding membranematerial; and swelling of the polymeric membrane as it sorbs the water.There may also be a degree of actual polymer solution, such as Stamm hasdocumented for wood and other cellulosic materials. Wood Sci. Technol. 3(1969), 301.

Exploratory measurements showed that all of the present series ofmembrane samples, treated and untreated, were so hydrophilic thatmeasurement of advancing and receding contact angles was impossible.This corresponds to the experience of Lloyd et al. with sulfonatedpolysulfone membranes. Annual Report to Office of Water Research andTechnology, April 1980. Therefore, it was decided to note initialcontact angles and also to follow the change in contact angle as afunction of time.

No possibility exists for obtaining an "equilibrium-contact-angle" valuein these systems. An "instantaneous contact angle" was obtained as afunction of time, anticipating that the slopes of these experimentalcurves will furnish bases for both comparison and interpretation.

So that a water drop would experience some of the same environment thatit would see if the membrane were part of an operating electrodialysisstack, its contact angle has measured on a "damp" sample. This procedurewould be more reliable if it were carried out in a chamber with 100%relative humidity, which is possible when an environmental chambersurrounds the contact-angle goniometer.

The sample was removed from Milli-Q storage and briefly patted betweenpaper towels. A standard drop of Milli-Q water was applied from asyringe fixed above the stage of the goniometer. The contact angle wasread as quickly as possible, and at regular intervals thereafter.

Changes visible at the drop-membrane junction follow the patterndiagrammed in Steps A through C of FIG. 10. Step A represents theappearance of the system immediately after the water drop is applied.Within 30 seconds, a haze appears at the junction. Later events indicatethat the boundary of this haze (represented by a dotted line in FIG. 10)is indeed the surface of the membrane, which is swelling as it sorbswater but is still too "dilute" to appear dark in the telescope.

At Stage C, contact-angle decay has reached a point where an angle isbarely measurable, and the drop is virtually completely sorbed. Thecurved surface of the swelling membrane appears dark in the telescope.

1. Untreated controls

Water drops at the surfaces of untreated AMF A-63 anion-exchangemembranes were completely sorbed in about 60 min.

2. Membranes coated with arachidic acid

Three (3) y-layers of arachidic acid, deposited at 25° C. and 25 mN M⁻¹,markedly enhanced the sorption process. A standard water drop was sorbedwithin 36 min.

3. Membranes coated with R_(f) PyrBr

FIGS. 11-13 illustrate the unusual behavior of water drops at thesurfaces of membranes treated with R_(f) PyrBr. In every case, the timerequired for drop disappearance was lengthened beyond the periodobserved for untreated controls. Furthermore, in the cases of samplescovered by three multilayers, the prolongation was roughly proportionalto the increase in the surface pressure at which the layers weredeposited. These observations are in line with the hypotheses on whichthe present invention was based.

The slope changes shown in FIGS. 11-13 have been observed with membranesmodified by R_(f) PyrBr. They do not, in one sense, represent the actualprogress of the wetting process, which was schematically illustrated inFIG. 10, A through D. It is evident that the amount of non-sbsorbedwater is greater in 10C than in 10D, but the "apparent contact angle" islarger in 10D. The swelling membrane has reached a plateau, bringing thebaseline, to which the contact angle is referred, nearly back to thehorizontal. At the same time, it was routinely found necessary torelocate the water drop in the telescope of the contact-anglegoniometer. The drop's shift in position is accompanied by an apparentcoalescence, which reduces its area of contact with the swollensubstrate. The baseline plateau and reduced contact area of the dropboth contribute to an abrupt increase in apparent contact angle, and aninflection point on the decay curve.

The wetting behavior of Sample 5 (FIG. 13), which was coated with threex-layers of R_(f) PyrBr at 40 mN M⁻¹ and 10.5° C., differed from theother R_(f) PyrBr systems. Sudden decline from an initially high contactangle was followed by stabilization at an angle of about 65°. This canbe termed a pseudo steady-state condition because the drop, in reality,underwent several incidents of coordinated baseline and contact areashifts such as described above. Thus, wetting, absorption, and dropdisappearance were in fact occurring, but are reflected in FIG. 13 onlyas small variations around an average steady-state angle. In thisinstance, interpretation of gross data responses without appreciation ofthe more subtle evidences of change in the system could lead to seriouserror.

After a span of 75 min. with only slight decreases in apparent contactangle, the water drop disappeared rapidly into the membrane. This was asingle experiment with this type of membrane.

Surprisingly, but in line with the SEM observations, infra, the 10-layerR_(f) PyrBr coating did not lead to further enhanced hydrophobicity(FIG. 13, Sample 6). The SEM of this sample indicated a high degree ofdisorder, which is consistent with more rapid wetting than observed withthe membrane coated by three layers of R_(f) PyrBr.

F. Scanning Electron Microscope Examination

1. Sample preparation

The earliest SEM studies were made on samples that had been imbibed witha glycerol-water mixture, vacuum dried, and sputtered with gold andplatinum. The imbibition step was incorporated as a means of keepingpores open, and metallic sputtering ensured sufficient surfaceconductance to give a clear picture.

It was found, however, that both procedures added experimental artifactsthat obscured rather than enhanced the significant characteristics ofthe modified membrane surfaces. Vacuum drying caused the glycerolmixture to attempt escape, forming blisters and pockets. Sputtering withmetals blanketed the much thinner deposited multilayers. If it had beenmandatory to sputter to obtain clear micrographs, this disadvantagewould have to have been accepted and allow for it in the interpretation.It was found, however, that the membranes themselves have sufficientcharge density to yield excellent SEMs, and that sputtering is totallyunnecessary. Indeed, the best pictures were obtained when the SEMpotential was reduced from 10.0 KV to 2.5 KV.

In the refined sample preparation, Milli-Q water was vacuum dried fromthe membrane. Vacuum drying itself may be inducing some collapse withinthe multilayer structure, but this part of the procedure is integral toSEM examination. The micrographs give little or no indication ofcollapse. This possibility must, however, be kept in mind in comparingthese scans with the results of fouling evaluations for membranes thathave been kept wet since their preparation.

2. Controls

An untreated sample of AMF A-63 anion-exchange membrane was used as thecontrol. Considerable debris was obvious on the surface, and the surfacecomposition itself appeared to be highly inhomogeneous. No cracks orpores were visible, which was also the case for the samples that wereimbibed with glycerol before examining.

G. Tests of ED-Membrane Fouling Propensities

1. Shakedown runs

Several accelerated fouling runs were carried out that lasted from 146to 186 hours, and others that were terminated after 20 hours. Except inone instance, which can be reasonably explained, the stack reached asteady-state current density before 20 hours had elapsed. No additionalinformation could be obtained by extending the experiments, and all runswere limited to a standard 20-hour period.

Because most of the fouling experiments were carried out before thelimiting current of the stack could be evaluated, an arbitrary choicewas made of operating potential. It was found that 2.0 volts, theconstant potential used throughout these tests, was considerably belowthe potentials required to produce limiting currents in this system. Forready comparison with large-scale industrial desalination conditions,operating potentials giving rise to 70% of the limiting current wouldhave been preferable. Nevertheless, these runs are valid fordemonstrating the effects of different modifications uponfouling-induced resistance increases.

The humate solutions were more concentrated than natural waters by afactor of 10⁴. Therefore, a 20-hour run exposed the membranes to amountsof humic acid many times greater than would ever be encountered duringsuccessive normal lifetimes in an electrodialysis stack. Of course,there are many factors in membrane deterioration other than exposure tohumates, and service-lifetime studies should be included in futureinvestigations of these membrane modifications.

The first set of shakedown runs was carried out at a constant potentialof 2.0 V, linear stream velocities of 0.68 cm/sec, and solutionconcentrations of 0.001N, with 0.1% (w/w) sodium humate added to the KClstream. When untreated AMF A-63 control membranes were mounted in thetest positions, the operating current densities fell from 17.4×10³¹ 3milliamps/cm² to 11.6 mA/cm² over the first hour and to 6.8×10⁻³overnight. For similar periods, the cell containing test membranescoated on one side by three layers of R_(f) PyrBr at 25° C. and 25 mNM⁻¹, exhibited currents of 13.6×10.3⁻³, 10.7×10⁻³, and 8.7×10⁻³ mA/cm².When membranes asymmetrically coated by three layers of arachidic acidwere installed, the initial current density, at the constant potentialof two volts, were 7.8×10⁻³ mA/cm² much lower than we observed with theother two membrane types. This indicates that the three (3) arachidicacid layers, with a total thickness of only 60 Å, produced a largeresistance increase.

2. Membranes modified by fluorinated pyridinium bromide

Six fouling runs were carried with membranes modified by Blodgettmultilayers of R_(f) PryBr. Results are shown below in Table III. Exceptfor two pairs, these membranes were modified under conditions differingin too many variables to yield truly reliable progressions.

                                      TABLE III                                   __________________________________________________________________________    FOULING OF AMF A-63 MEMBRANES MODIFIED BY                                     FLUORINATED PYRIDINIUM BROMIDE LAYERS                                         Type of                                                                            Number                                                                             Number                                                                             .sup.π depo-                                                                    .sup.τ depo-                                                                  Condition  .sup.a R. steady-                                                                   R. steady-                                                                          ΔR.                                                                        Period                      treat-                                                                             of sides                                                                           of layers                                                                          sition,                                                                            sition,                                                                           at    R. initial,                                                                        state state kilo-                                                                            of test,                    ment treated                                                                            applied                                                                            dynes/cm                                                                           °C.                                                                        deposition                                                                          kilo-ohms                                                                          kilo-ohms                                                                           R. initial                                                                          ohms                                                                             hours                       __________________________________________________________________________    Control                                                                            --   --   --   --  --    17.8 45.7  2.6   27.9                                                                              17                         Control                                                                            --   --   --   --  --    20.0 52.6  2.6   32.6                                                                             186                         R.sub.∫ PyrBr                                                                 1    1    40   10.5                                                                              dry   14.5 17.8  1.2   3.3                                                                              183                         R.sub.∫ PyrBr                                                                 1    .sup. 2X.sup.b                                                                     40   10.5                                                                              dry   26.7 40.0  1.5   13.3                                                                              25                         R.sub.∫ PyrBr                                                                 2    3X   35   10.5                                                                              dry   26.7 40.0  1.5   13.3                                                                             146                         R.sub.∫ PyrBr.sup.c                                                           1    .sup. 3Y.sup.d                                                                     25   25.0                                                                              dry   22.9 35.6  1.6   12.7                                                                              20                         R.sub.∫ PyrBr                                                                 2    3X   35   10.5                                                                              wet   22.7 57.1  2.5   34.4                                                                             186                         R.sub.∫ PyrBr                                                                 1    10X  40   10.5                                                                              dry   25.0 40.0  1.6   15.0                                                                              30                         __________________________________________________________________________     .sup.a Total resistance of stack assembly with all test membranes             .sup.b Blodgett Xmultilayers, headtail-head-tail pattern                      .sup.c This set of membranes has been stored 2 months at room temperature     .sup.d Blodgett Ymultilayers, headtail-tail-head-pattern                 

With the exception of one membrane set modified while wet, all R_(f)PyrBr-coated membranes, compared to untreated controls after both typeswere fouled, exhibited lower resistances. One layer of R_(f) PyrBr,deposited on a dry membrane at 40 dynes/cm and 10.5° C. (the thirdsample in Table III) had two effects on resistance: it reduced initialmembrane resistance by almost 20%, and it reduced the relativeresistance rise on fouling to 46% of the increase for untreatedcontrols. The actual resistance change due to fouling was reduced from30 to 3.3 kilo-ohms. The fouling test samples were always kept wet,affording the modifying layers optimal conditions for retention of theintegrity of freshly prepared samples.

By comparison, 2x-layers of R_(f) PyrBr (the fourth sample in Table III)and 3x-layers of R_(f) PyrBr (the fifth sample in Table III), depositedat 40 dynes/cm and 35 dynes/cm, respectively, raised the initialresistance by 33%, but also reduced the resistance rise on fouling to58% of that experienced by untreated controls. However, the steady-stateresistance reading was 81% of the steady-state resistance of untreatedmembranes, more than twice the steady-state resistance of the samplecoated by one monomolecular layer of R_(f) PyrBr.

X-layers were formed on these samples by vacuuming excess R_(f) PyrBrfilm from the water surface while the dipped membranes, coated by priorlayers, were still submerged. Emersion then occurred through a cleansurface, and a fresh R_(f) PyrBr film was spread before the nextimmersion. Each immersion resulted in highly reproducible float movementin the dipping trough, which indicated the transfer of a highly regularfilm well registered with the substrate. This arrangement of layers isapparently the preferred configuration for R_(f) PyrBr, giving the bestmultilayer packing.

It is therefore surprising that 3 y-layers of R_(f) PyrBr, deposited at25° C. and 25 dynes/cm (the seventh sample in Table III) were almost assuccessful as 1, 2, 3, and 10 x-layers in reducing resistance increasesduring fouling. It was observed earlier that the π-A curves of R_(f)PyrBr showed that the film at a surface pressure of 25 dynes/cm is inonly a slightly condensed state, which might hinder transfer to asubstrate. However, an SEM, at magnification 1000X, of a membrane coatedwith R_(f) PyrBr at 25 dyne/cm exhibited a distinctive appearancerelative to an untreated surface. The SEM makes it obvious that thepreviously inhomogeneous membrane surface has been covered by a filmthat is coherent except for pores that can be ascribed to layer collapseduring vacuum drying.

A likely explanation of the reduction in resistance caused by thesey-layers lies in the fact that the seventh sample of membranes in TableIII had been coated with R_(f) PyrBr two months before the fouling test,and they had been stored at room temperature. Materials deposited in anarrangement different from the preferred pattern have a tendency torearrange to the preferred pattern over a period of time, whileretaining the multilayered configuration. Fort et al. used X-raydiffraction to discern this behavior in aging multilayers of ethylstearate. J. Polym. Sci. Part A-1 10 (1972), 1061. It is possible thatthe y-layers originally deposited rearranged to x-layers during theextended interval between preparation and fouling. Their influence onelectrical resistance would then be similar to the effect of multilayersoriginally deposited in the x-pattern.

The seventh test run was the only evaluation during which the test cellexperienced a further resistance increase after 20 hours. In that case,the resistance was steady at 57.1 kilo-ohms (the value reported in TableIII) for 100 hours or longer and then underwent a rise toward 65kilo-ohms at 185 hours. This result is interpreted as due to partialcoverage of the substrate membrane by poorly attached R_(f) PyrBrmonolayers. It was speculated that, on undergoing a 20% change indimensions as they sorbed water, membranes that were coated while drymight disturb the continuity of the deposited surface layers. If so,attachment of layers to a prewetted membrane would improve the coherenceof the coating.

During application of successive monolayers to the water-swollenmembranes, it was noted that float movement in the Blodgett-Kuhn troughwas extremely erratic. Total movement during one immersion was only afraction of that undergone during normal dipping of a dry substrate.These observations made both coverage and attachment of the modifyingfilms questionable. The results of the fouling tests confirmed thesuspicion that preswollen membranes cannot satisfactorily acceptBlodgett layers.

3. Membranes modified by arachidic acid

In all cases of membranes modified by layers of arachidic acid,resistances doubled during the course of fouling-evaluation runs.Furthermore, the initial resistance exhibited by an ED stack containingmembrane samples coated with 1, 2, 3 or 10 y-layers were identicallytwice that of a stack with untreated control membranes. It appears thatone layer, although it is only 20 Å thick, is sufficient to provide"pre-fouling" that the addition of more arachidic acid layers does notsupplement.

Resistence increases for membranes coated with arachidic acid wereconsiderably greater than for membranes coated by R_(f) PyrBr, and alsolarger than exhibited by untreated controls. The SEM observations implythat, once fouled by an oriented layer, the membranes gained surfacehomogeneity that should somewhat reduce their propensity for continuedfouling. It is evident, however, that the charge disparity between theoriented layer and the substrate, which Korngold et al. dubbed the"sandwich effect," is of overwhelming importance in determining foulingpropensities.

4. Light transmission of fouled membranes

On visual examination of the fouled membranes, no differences incoloration could be detected between their modified and unmodifiedsides. The unmodified sides, however, displayed a dull patina, whereasthe treated sides were glossy. These differences in appearance indicatethat humic acids have occluded the untreated sides, but not the treatedsides. Humic acid coloration was homogeneous throughout the samples, butwas less intense overall in the membranes coated with R_(f) PyrBr.Because, for accelerated fouling tests, the treatment solution containedmuch more sodium humate than would ever be found in natural waters,significant amounts of humic acid may adhere to the untreated sides ofthe membranes by concentration-driven adsorption.

Observations confirming this were made on membranes modified by R_(f)PyrBr and by arachidic acid. One membrane of each pair confronted thehumate solution with a multilayered surface, while the other confrontedit with the untreated surface. For both types of treatment (R_(f) PyrBrand arachidic acid), the membrane with a treated surface facing humatesretained much less coloration than its mirror image. The controlmembranes exhibited about the same humic acid pickup in both positions.

All fouled membranes, treated and untreated, were stained brown, and thedepth of color varied with the position of the membranes in theelectrodialysis cell. The observations indicate that treatment withR_(f) PyrBr had no effect on the total amount of humates that adhere tothe membrane, but that, at least in this case, multilayer coatingprevented the humates from being irreversibly adsorbed by the substratematerial.

V. SUMMARY OF RESULTS

It is possible to construct negatively charged or positively chargedmultilayer assemblies on the surfaces of anion-exchange membranes,thereby modifying the surfaces.

These layers, when assembled at optimal temperature and pressureconditions, confer a marked degree of microscopic homogeneity on thesurfaces.

The wetting characteristics of the membranes can be altered by additionof oriented multilayers. In the case of layers with a charge opposite tothat of the substrate, the sorption time for a standard drop of water isshortened; if the layers are fluorinated and have the same charge as themembrane, the sorption time is prolonged.

Membranes modified by oppositely charged multilayers wet more quicklythan untreated controls.

Membranes modified by R_(f) PyrBr layers wet twice as slowly asuntreated controls.

There is a correlation, in the case of three layers of fluorinatedmaterial with the same charge as the substrate, between the tightness ofpacking in the layers and the lengthening of the wetting time.

Both types of multilayer treatment raised the Cowan-method limitingcurrent of anion-exchange membranes, relative to i_(lim) of untreatedcontrols.

However, the effects upon power requirements of the membranes whenelectrolysis was carried out in the operating range (70% i_(lim)) of anultra-clean system were dramatically different. Power requirements formembranes coated by oppositely charged layers were multiplied by 9.Power requirements for membranes coated by fluorinated layers with likecharges were multiplied by 3.

The initial resistance of membranes asymmetrically coated by oppositelycharged layers was high relative to that of the controls. It becamestill higher during operation of an electrodialysis system loaded with10⁴ times a natural level of humates. This behavior implies that thefirst few molecular layers added to a substrate during fouling have themost drastic effect upon its electrodialytic properties.

The actual resistance increase during fouling tests is much greater formembranes treated by oppositely charged monolayer assemblies than foruntreated controls; the percentage increase over the initial value islower.

Membranes modified by oppositely charged monolayer assemblies sorb morehumate color during a fouling run than untreated controls if theoutermost layer is nominally polar; they sorb approximately the sameamount if the outer layer is nominally nonpolar.

The alterations caused by oppositely charged monolayer assemblies inboth wetting and fouling behavior indicate that homogeneity of thesurface exerts an influence on these phenomena that is negligiblerelative to the influences of charge disparity and hydrophobicity.

The examination of pairs of membranes from the fouling stackdemonstrates that treatment by multilayering with either similarly oroppositely charged materials interferes with the deposition of humatesat a membrane surface.

Judging from visual examination, all of the membranes, including theasymmetrically modified samples, adsorbed significant amounts of humatesduring the fouling evaluations. Thus, although the modifying layers mayhave prevented precipitation of partially neutralized humates at thesides of the test samples that faced the cathode, they did not prevententry of unneutralized colloidal material from the sides facing theanode.

Membranes modified by fluorinated similarly charged monolayers exhibitedan electrical resistance prior to humate fouling that is almost the sameas the resistance of fouled control samples.

Actual electrical resistance increases for membranes modified by R_(f)PyrBr x-layers were small compared to those of untreated controls;percentage increases of fouled over initial resistance were halved whenthese layers were present. R_(f) PyrBr X-layers cause membranes to sorbmuch more humate color during a fouling run than was sorbed by untreatedcontrols.

Treatment with one layer of R_(f) PyrBr at 40 mN M⁻¹ and 10.5° C. wasthe most successful anti-fouling preventive tested, cutting actualresistance increase from 30 to 3.3 kilo-ohms and the ratio of final andinitial resistances from 2.6 to 1.2.

Correlation of Results and Theory

The experiments have borne out the hypotheses that even a singledeposited oriented monolayer, with a thickness of 20 Å, strikinglymodified both the microscopic appearance and the electrical and foulingbehaviors of anion-exchange membranes. While the effects upon appearancewere similar, the effects upon wetting behavior and resistance changesduring accelerated fouling tests were opposite for layers charged likeand unlike the substrate membrane. Therefore, surface roughness andinhomogeneity would appear to be minor factors in the fouling process.

A single monolayer of a fluorinated pyridinium bromide cut resistancerises due to fouling by a factor of 9, demonstrating that thismodification holds promise of greatly improving the economics ofelectrodialytic desalination.

VI. REVERSE OSMOSIS MEMBRANES COATED WITH FLUORINATED PYRIDINIUM BROMIDE

Oriented deposition of 1 Blodgett layer of R_(f) PyrBr was used tomodify the surfaces of two types of commercial cellulose acetatereverse-osmosis (RO) membranes. The dipping pressure was 40 mN M⁻¹, andthe temperature of the system was maintained at 10.5° C. One type ofsubstrate membrane was obtained from Hydranautics, Inc., the other fromFluid Systems, Inc.

By the following table, it can be seen that one layer of R_(f) PyrBrreduced the throughput of the Fluid Systems membrane almost to zero, butit had very little effect on the throughput of the Hydranautics sample.It is probable that the high initial flux of the Fluid Systems membraneis due to the cracks that can be detected in its SEM. These cracks,which would also lead to undesirably low salt rejection, were sealed byapplication of one monolayer of R_(f) PyrBr.

                  TABLE II                                                        ______________________________________                                        EFFECT OF MONOLAYERING ON THROUGHPUT                                          OF RO MEMBRANES.sup.a                                                                                Test Period.                                                                             Throughput.                                 Membrane  Modification hours      gfd.sup.b                                   ______________________________________                                        Hydranautics                                                                            None         16         2.71                                        Hydranautics                                                                            1 layer R.sub.∫ PyrBr                                                                 18         2.24                                        Fluid Systems                                                                           None         10         4.29                                        Fluid Systems                                                                           1 layer R.sub.∫ PyrBr                                                                 19         0.06                                        ______________________________________                                         .sup.a Exposed to 0.1 molar NaCl containing 2 ppm sodium humate at 160        psi, 25° C.                                                            .sup.b Gal/ft.sup.2 · day                                       

Thus, monolayering treatment apparently reversed a surfacecharacteristic of the Fluid Systems membrane that would be a source ofundesirable operating properties; the monolayer simultaneously reducedtransmembrane flux. The Hydranautics membrane, which exhibited no cracksin its unmodified state, experienced only a minor reduction in flux.

Long-term fouling experiments are needed to compare flux reduction bymonolayering with flux reduction by foulant buildup. The preliminaryobservations indicate that a membrane with high salt rejection andreasonable transmembrane flux will retain these properties aftermonolayering, but will also tend to repel foulants.

What I claim is:
 1. A method of modifying a surface of a separatorymaterial comprising the step of depositing on said surface .[.a.]..Iadd.at least one oriented monomolecular .Iaddend.layer of .[.a.]..Iadd.an essentially water-insoluble .Iaddend.fluorinated amphiphilicmolecule .Iadd.which is capable of forming an oriented monomolecularlayer and .Iaddend.which is oriented such that the fluorinated portionof said molecule extends outwardly from said surface.
 2. A method asclaimed in claim 1 wherein said molecule is a fluorinated aromaticheterocyclic bromide.
 3. A method as claimed in claim 1 wherein saidmolecule is fluorinated pyridinium bromide.
 4. A method as claimed inclaim 1 wherein said layer is a monomolecular Langmuir-Blodgett layer.5. A method as claimed in claim 1 wherein said molecule is neutral incharge.
 6. A method as claimed in claim 1 wherein said molecule has thesame electrical charge as said membrane surface.
 7. A method as claimedin claim 1 wherein said layer of said molecule is deposited on saidsurface at a deposition pressure ranging from 30 to 35 mN M⁻¹.
 8. Amethod as claimed in claim 1 wherein said layer of said molecule isdeposited on said surface at a deposition temperature ranging from 1° to10° C.
 9. A method as claimed in claim 1 wherein the molecular weight ofsaid molecule ranges from 350 to
 700. 10. A method as claimed in claim 1wherein said molecule is water-insoluble at a deposition pressureranging from 30 to 35 mN M⁻¹ and a deposition temperature of 1° to 10°C.
 11. A method as claimed in claim 1 wherein said separatory materialis a semipermeable membrane.
 12. A method as claimed in claim 11 whereinsaid membrane is a liquid-liquid separatory membrane.
 13. A method asclaimed in claim 11 wherein said membrane is a gas-gas separatorymembrane.
 14. A method as claimed in claim 1 wherein said separatorymaterial is a resin particle.
 15. A method of treating the surfaces ofsemipermeable membranes which separates a component of a fluid,comprising the steps of:(a) placing .Iadd.on the surface of a body ofwater .Iaddend.a film of molecules of .[.a.]. .Iadd.an essentially waterinsoluble .Iaddend.fluorinated amphiphilic compound .[.on the surface ofa body of water.]. .Iadd.which is capable of forming an orientedmonomolecular layer.Iaddend.; (b) compressing said film into an orientedmonomolecular layer; and (c) moving said membrane vertically into andout of said film spread on a water surface a selected number of timeswhereby said monolayer is transferred onto an exterior surface of saidmembrane such that the fluorinated portion of said compound is directedtoward said fluid.
 16. A method as claimed in claim 15 wherein saidcompound is fluorinated pyridinium bromide.
 17. A method of enhancingthe selectivity of a selective semipermeable membrane for a component ofa mixture introduced to a surface of said membrane, comprising the stepof depositing on said surface .[.an.]. .Iadd.at least one.Iaddend.oriented monolayer of .[.a.]. .Iadd.an essentially waterinsoluble .Iaddend.fluorinated amphiphilic compound .Iadd.which iscapable of forming an oriented monomolecular layer and .Iaddend.whichexhibits an affinity for said component.
 18. A method as claimed inclaim 17 wherein said compound is surface-active.
 19. A method ofmodifying the surface of a semipermeable membrane comprising the step ofdepositing on said surface .[.an.]. .Iadd.at least one .Iaddend.orientedmonolayer of .[.a.]. .Iadd.an essentially water insoluble.Iaddend.fluorinated amphiphilic compound .Iadd.which is capable offorming an oriented monomolecular layer and .Iaddend.which has the samecharge as said surface.
 20. An improvement in a semipermeable membraneof the type having a surface in contact with a fluid to be separated,the improvement comprising .[.a.]. .Iadd.an essentially water insoluble.Iaddend.fluorinated amphiphilic compound .Iadd.capable of forming anoriented monomolecular layer .Iaddend.being deposited on said surface in.[.an.]. .Iadd.at least one .Iaddend.oriented monomolecularLangmuir-Blodgett layer so as to impart non-fouling characteristics tosaid membrane.
 21. An improvement as claimed in claim 20 wherein saidcompound is fluorinated pyridinium bromide.
 22. An improvement asclaimed in claim 20 wherein said layer is oriented with the fluorinatedportion of said compound directed toward said fluid to be separated. 23.An improvement as claimed in claim 20 wherein said layer is an x-layerof fluorinated pyridinium bromide.
 24. A method of preventing thefouling of a semipermeable membrane used for separating dissolvedmaterials from liquids, comprising the step of depositing on the surfaceof said membrane .[.an.]. .Iadd.at least one .Iaddend.orientedmonololecular layer of fluorinated pyridinium bromide prior to the useof said surface in said separating.
 25. An improvement in asemipermeable membrane of the type used for separating dissolvedmaterials from liquids, the improvement comprising fluorinatedpyridinium bromide being deposited in .[.an.]. .Iadd.at least one.Iaddend.oriented monomolecular layer on the surface of said membranethat is to be in contact with said liquid. .Iadd.
 26. A method ofmodifying a solid surface of a material comprising the step ofdepositing on said surface at least one oriented monomolecular layer ofan essentially water-insoluble fluorinated amphiphilic compound which iscapable of forming an oriented monomolecular layer and which is orientedsuch that the fluorinated portions of the molecules of said compoundextend outwardly from said surface. .Iaddend. .Iadd.27. A method asclaimed in claim 26 wherein each layer of said fluorinated amphiphiliccompound is a monomolecular Langmuir-Blodgett layer. .Iaddend. .Iadd.28.A material having improved non-fouling characteristics, said materialincluding a smooth solid surface having thereon at least one orientedmonomolecular layer of an essentially water-insoluble fluorinatedamphiphilic compound capable of forming an oriented monomolecular layer,said compound imparting improved non-fouling characteristics to saidsurface. .Iaddend. .Iadd.29. A material according to claim 28 whereinsaid layer is a Langmuir-Blodgett layer. .Iaddend. .Iadd.30. A methodaccording to claim 26 wherein said compound is a long chain compoundhaving a fluorinated group and a polar group at opposite ends of saidchain, said compound being neutral in charge or having the same chargeas that of said surface. .Iaddend. .Iadd.31. A method according to claim26 wherein said surface is hydrophilic. .Iaddend. .Iadd.32. A materialaccording to claim 28 wherein said compound is a long chain compoundhaving a fluorinated group and a polar group at opposite ends of saidchain, said compound being neutral in charge or having the same chargeas that of said surface. .Iaddend.